The Na symporter (NIS) is a plasma membrane protein that mediates active I- transport in the thyroid and other tissues, including salivary glands, gastric mucosa, and lactating mammary glands (MG). NIS is regulated differently in each tissue. NIS-mediated I- transport in the thyroid is the first step in thyroid hormone biosynthesis. Endogenous functional expression of NIS in thyroid cancer is the foundation for the single most effective and most side effect-free anti-cancerous targeted radiation therapy available, i.e. radioiodide therapy, which has been successfully used in thyroid cancer for over 60 years. Our group isolated the cDNA encoding NIS and generated anti-NIS Abs. We have characterized thyroid NIS and its regulation. Mammary gland NIS (mgNIS) mediates active I- transport in lactating mammary cells, from which I- is passively translocated via a different transporter to the milk. mgNIS is expressed in lactating (but not in non-lactating) MG. mgNIS is regulated by estrogen, prolactin, and oxytocin. Mammary adenocarcinomas in transgenic mice display mgNIS-mediated active I- uptake. The only two cancers in which endogenous functional NIS is expressed are thyroid cancer and breast cancer. Over 80% of human breast cancers express mgNIS, but it is still unknown in what percentage of these NIS is functional. Normal non-lactating human breast samples do not express mgNIS. The endogenous expression of NIS in breast cancer provides an immense advantage over other cancers, into some of which NIS has been ectopically expressed by virus-mediated gene transfer. To characterize mgNIS in mammary cells and ascertain its potential value in breast cancer diagnosis and treatment, we propose: 1. a) To characterize the regulation of mgNIS in mammary cell lines; b) to investigate the effects of systemic and local regulatory factors on the expression of mgNIS in vivo; c) to determine whether low temperature and chemical chaperones promote targeting of mgNIS to the plasma membrane; d) to complement our understanding of I- translocation in the lactating mammary gland by assessing the role of putative apical I- transporters. 2) To develop a radioiodide therapeutic protocol and assess its effectiveness in the treatment of adenocarcinomas in animal models. 3) To ascertain, in fine needle aspirates from both human primary breast tumors and metastases, the functional expression of mgNIS and its possible correlation with other breast cancer parameters.